Electric power steering control system

ABSTRACT

Intended is to solve the problem of an increase in the cost of an electric power steering control system, the oscillations of which are suppressed by estimating and feeding back the oscillation frequency components of a motor rotating speed through an observer from a steering torque signal and a current signal for driving a motor, no matter whether a phase compensator might be made of an analog circuit or a software. In order to solve this problem, there is provided the electric power steering control system, in which the phase compensator of the steering torque is made of an analog circuit and in which an anti-phase compensator is made over the software of a microcomputer, thereby to eliminate the changes in the gain and the phase by the phase compensator of the analog circuit near the oscillation frequency, so that the steering torque signal equivalent to that of no phase compensation necessary for the computation at the observer is computed from the phase-compensated steering torque signal.

TECHNICAL FIELD

The invention relates to an electric power steering control system forautomobiles.

BACKGROUND ART

In electric power steering systems, an assist torque substantiallyproportional to a steering torque is determined. The steering torque ofa driver of an automobile is reduced by increasing a torque proportionalgain that corresponds to the proportional relationship. At this time, ifthe torque proportional gain is too large, a control system oscillatesto bring about steering wheel oscillations. In some cases, the degree towhich the steering torque is reduced is therefore restricted. In orderto solve this problem, an algorithm for suppressing oscillations byimproving the phase characteristic of the control system throughintroduction of a phase compensator has been invented to prevent thesteering wheel oscillations (refer to, for example, a referenceliterature 1).

For the purpose of improving oscillation suppression performance, inaddition to the phase compensator, a control algorithm that suppressesoscillations by inferring an oscillation frequency component of a motorrotating speed from a steering torque signal and a current signal, whichdrives a motor, by means of an observer, and feeding back theoscillation frequency component has been invented to prevent steeringwheel oscillations (refer to, for example, a reference literature 2).

Patent document 1: JP-A-8-91236 (p. 4, FIG. 1)

Patent document 2: JP-A-2000-168600 (p. 10, FIG. 12)

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In the electric power steering system, a majority of the controlalgorithm is constituted by software, the steering torque signal isfetched into a microcomputer through a torque signal converter, that is,an A/D conversion circuit, and then arithmetically processed. This isbecause the oscillation frequency in electric power steering isrelatively low to range from 30 Hz to 100 Hz. However, there are a casewhere the phase compensator alone included in the control algorithm isformed by software and the microcomputer is used for computation, and acase where an analog circuit directly performs phase compensation. Whenthe microcomputer is used for computation, if the oscillation frequencyof the control system is high, high-speed arithmetic processing isrequired. An expensive microcomputer is therefore needed.

When the analog circuit performs phase compensation, since a steeringtorque signal on which phase compensation is not performed is needed forthe computation by the observer, both a steering torque signal on whichphase compensation is performed by the analog circuit and a steeringtorque signal on which phase compensation is not performed have to beA/D-converted and fetched into the microcomputer. The number of requiredA/D conversion circuits therefore increases.

As mentioned above, when the control algorithm that suppressesoscillations by inferring an oscillation frequency component of a motorrotating speed from a steering torque signal and a current signal, whichdrives a motor, by means of an observer, and feeding back theoscillation frequency component is used in addition to the phasecompensator, whether the phase compensator is formed by an analogcircuit or software, the cost increases.

The invention is intended to solve the foregoing problems and to providean electric power steering control system that realizes excellentoscillation suppression performance without increasing the number of A/Dconversion circuits, that is, without inviting an increase in the cost.

Means for Solving the Problems

An electric power steering system in accordance with the inventionincludes: a torque detection means that detects a steering torque causedby a driver; an analog phase compensator that performs phasecompensation on an output of the torque detection means by means of ananalog circuit; a torque signal converter that A/D-converts an output ofthe analog phase compensator and fetches the resultant output into amicrocomputer; a torque controller that computes an auxiliary torquecurrent, which assists the steering torque, using the output of thetorque signal converter; a motor that generates a torque which assiststhe steering torque; a current detection means that detects a currentvalue to be conducted to the motor; a rotating speed estimation meansthat estimates the rotating speed of the motor; and a damping controllerthat computes a damping current, which is added to the auxiliary torquecurrent, using an estimate value of the motor rotating speed estimatedby the rotating speed estimation means. The rotating speed estimationmeans includes a rotating speed observer that computes the estimatevalue of the motor rotating speed using the output of a torque oppositephase compensator that adjusts the phase and gain of the output of thetorque signal converter, and the output of the current detection means.

ADVANTAGE OF THE INVENTION

According to the invention, the phase compensator for a steering torqueis formed with an analog circuit, and the opposite phase compensator isformed by software in the microcomputer. The changes in a gain and aphase caused by the phase compensator of an analog circuit are canceledat an oscillation frequency. A steering torque signal equivalent to asteering torque signal that is needed for computation by the observerand has not undergone phase compensation is computed from a steeringtorque signal having undergone phase compensation. Since this obviatesthe necessity of A/D-converting the steering torque signal that has notundergone phase compensation, and fetching the resultant signal into amicrocomputer, the number of A/D conversion circuits decreases.Eventually, excellent oscillation suppression performance can berealized without an increase in the cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the configuration of a controller inan embodiment 1;

FIG. 2 includes Bode diagrams showing the frequency characteristics ofan opposite phase compensator of the controller in the embodiment 1;

FIG. 3 is a flowchart presenting processing to be performed in amicrocomputer of the controller in the embodiment 1;

FIG. 4 includes Bode diagrams showing the frequency characteristics ofan opposite phase compensator of a controller in an embodiment 2;

FIG. 5 is a block diagram showing the configuration of a controller inan embodiment 3;

FIG. 6 includes Bode diagrams showing the frequency characteristics of atorque HPF of the controller in the embodiment 3;

FIG. 7 includes Bode diagrams showing the frequency characteristics of atorque phase lead reducer of the controller in the embodiment 3;

FIG. 8 is a flowchart presenting processing to be performed in amicrocomputer of the controller in the embodiment 3; and

FIG. 9 includes Bode diagrams showing the frequency characteristics of acombination of a torque HPF and a torque phase lead reducer of thecontroller in the embodiment 3.

DESCRIPTION OF REFERENCE NUMERALS

1: torque sensor, 2: analog phase compensator, 3: torque A/D converter,4: current detector, 5: current A/D converter, 6: torque controller, 7:torque opposite phase compensator, 8: torque HPF, 9: current HPF, 10:motor rotation observer, 11: damping controller, 12: adder, 13: currentcontroller, 14: drive circuit, 15: motor, 16: torque phase lead reducer,17: current phase lead reducer, 18: first rotating speed estimator, 19:second rotating speed estimator

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1

FIG. 1 is a block diagram showing the configuration of a controller inan embodiment 1 of the invention. A steering torque occurring when adriver performs steering is detected by a torque sensor 1, and an analogphase compensator 2 formed with an analog circuit improves a frequencycharacteristic so that a phase will most greatly lead at an oscillationfrequency f_(vib). A torque sensor output for which the frequencycharacteristic is improved is converted into a digital signal by an A/Dconverter 3, and fetched into a microcomputer. A current which drives amotor is detected by a current detector 4, converted into a digitalsignal by a current A/D converter 5, and fetched into the microcomputer.In the microcomputer, a torque controller 6 computes an auxiliary torquecurrent using the torque sensor output for which the frequencycharacteristic is improved. A torque opposite phase compensator 7computers an opposite characteristic of the analog phase compensator,and inputs the opposite characteristic to a first rotating speedestimator 18 (part of the drawing encircled with a dot line). The firstrotating speed estimator 18 estimates or computes a motor rotatingspeed, which has a low-frequency component thereof cut by a motorrotation observer (identical to a rotating speed observer) (the sameapplies to a description to be made below) 10, using a signal producedby cutting a low-frequency component of the steering torque signal, forwhich the frequency characteristic is returned to a frequencycharacteristic equivalent to that for a phase-uncompensated steeringtorque, through high-pass filter computation performed by a torque HPF8, and a signal produced by cutting a low frequency component of acurrent through high-pass filter computation performed by a current HPF9. A damping controller 11 computes a damping current using theresultant estimated motor rotating speed, and an adder 12 adds up anauxiliary torque current and the damping current so as to perform targetcurrent computation. A current controller 13 controls a current so thatthe computed target current and a current detected by the currentdetector 4 will be squared with each other. The resultant current isoutputted as a voltage command signal, for example, a PWM signal to adrive circuit 14, and a motor 15 is driven so that an assist torque willbe generated. Herein, the motor rotation observer 10 that performsestimation computation of a motor rotating speed is, for example, anobserver using as a model a single-degree-of-freedom oscillationequation that has an inertia moment of a motor as an inertial term and aspring constant of a torque sensor as a spring term (the same applies toa description to be made later). Moreover, in the invention, therotating speed estimation means encompasses as an example thereof thefirst rotating speed estimator.

As shown in FIG. 2, the characteristic B of the torque opposite phasecompensator 7 is determined to be opposite to the characteristic A ofthe analog phase compensator 2. Specifically, assuming that thecharacteristic A of the analog phase compensator 2 causes a gain toincrease within a frequency range from f_(a1) to f_(a2), f_(a1)=fi₁ andf_(a2)=f_(i2) are established so that a frequency range from f_(i1) tof_(i2) within which the characteristic B of the torque opposite phasecompensator 7 causes a gain to decrease will be identical to thefrequency range from f_(a1) to f_(a2). A total characteristic that is acombination of the characteristic A of the analog phase compensator 2and the characteristic B of the torque opposite phase compensator 7shall be a characteristic C.

Moreover, a maximum frequency to be cut by a high-pass filter that isthe torque HPF (where HPF stands for a high-pass filter) (The sameapplies to a description to be made below.) 8 or the current HPF 9 isset to a frequency higher than 5 Hz that is a maximum frequency which adriver can support.

Next, a processing algorithm in the microcomputer in the embodiment 1will be described in conjunction with the flowchart of FIG. 3.

First, at step S101, a torque sensor output that is A/D-converted by thetorque A/D converter 3 and is analog-phase-compensated is read andstored in a memory. At step S102, the current A/D converter 5 reads anA/D-converted current detection value (identical to a current detectoroutput) (the same applies to a description to be made below.), andstores it in the memory. At step S103, the torque controller 6 reads thetorque sensor output, which is stored in the memory and isanalog-phase-compensated, performs mapping computation on an auxiliarytorque current, and stores the resultant current in the memory. At stepS104, the torque opposite phase compensator 7 reads the torque sensoroutput, which is stored in the memory and is analog-phase-compensated,performs opposite phase compensation computation, and stores the torquesensor output, which has undergone opposite phase compensationcomputation, in the memory. At step S105, the torque HPF 8 performshigh-pass filter computation on the torque sensor output that is storedin the memory and has undergone opposite phase compensation computation,and stores the torque sensor output, which is high-pass-filtered, in thememory. At step S106, the current HPF 9 performs high-pass filtercomputation on the current detection value that is stored in the memory,and stores the current detection value, which is high-pass-filtered, inthe memory. At step S107, the motor rotation observer 10 performs motorrotating speed computation using the torque sensor output, which isstored in the memory and is high-pass-filtered, and the currentdetection value that is high-pass-filtered, and stores the result in thememory. At step S108, the damping controller 11 computes a dampingcurrent by multiplying the motor rotating speed, which is stored in thememory, by a gain, and stores the damping current in the memory. At stepS109, the adder 12 adds up the auxiliary torque current and dampingcurrent so as to obtain a target current. At step S110, the currentcontroller 13 performs current control computation using the targetcurrent and current detection value, and outputs the result as a voltagecommand signal such as a PWM signal to the drive circuit 14. Theprocessing from step S101 to step S110 is executed for every controlsampling.

According to the foregoing constitution, a torque sensor signal that isneeded for computation of an auxiliary torque current by the torquecontroller 6 and is analog-phase-compensated, and a torque sensor signalthat is needed for computation by the motor rotation observer 10 and isnot phase-compensated can be obtained with the one torque A/D converter3. Consequently, excellent oscillation suppression performance can berealized without an increase in a cost.

In the present embodiment, an electric signal to be inputted to thecurrent HPF 9 is detected by the current detector 4, and a currentdetection value obtained by converting the current signal into a digitalsignal by the current A/D converter 5 is employed. Alternatively, atarget current computed by the adder 12 may be employed.

Embodiment 2

The embodiment 2 is different from the embodiment 1 in the softwareconfiguration of the torque opposite phase compensator 7. The differencealone will be described using FIG. 4.

In the embodiment 2, the characteristic D of the torque opposite phasecompensator 7 is approximated to the characteristic of the analog phasecompensator 2. Specifically, assuming that the characteristic A of theanalog phase compensator 2 is a characteristic causing a gain toincrease at a ratio of, for example, 20 dB/dec within a frequency rangefrom f_(a1) to f_(a2), the torque opposite phase compensator 7 shall bea low-pass filter that causes a gain to begin decreasing at f_(i1) atthe ratio of 20 dB/dec, and f_(a1)=f_(i1) is established. When computinga damping current, the damping controller 11 needs a highly precisemotor rotating speed signal relative to a frequency at which steeringwheel oscillations occur. Consequently, the motor rotation observer 10needs a torque sensor output, which is devoid of a phase shift caused bythe analog phase compensator 2, relative to the frequency at which thesteering wheel oscillations occur. On the other hand, since f_(a1) andf_(a2) are determined so that the range from f_(a1) to f_(a2) willinclude the frequency at which steering wheel oscillations occur, evenwhen the torque opposite phase compensator is the low-pass filter thatcauses a gain to begin decreasing at f_(i1), the total characteristicthat is the combination of the characteristic A of the analog phasecompensator 2 and the characteristic D of the torque opposite phasecompensator 7 causes, as indicated with E, a gain to remain even andcauses a phase shift to diminish. Consequently, a higher-precision motorrotating speed signal can be obtained than that obtained when the torqueopposite phase compensator 7 is not included. As a result, the samesteering wheel oscillation reduction effect as that of the embodiment 1can be realized.

According to the foregoing constitution, since a simple low-pass filteris employed, the necessity of computing a characteristic opposite to thecharacteristic of the analog phase compensator 2 over a high-frequencydomain over which high-speed arithmetic processing is required isobviated. Consequently, the foregoing effect can be realized despite aninexpensive microcomputer.

Embodiment 3

FIG. 5 is a block diagram showing the configuration of a controller inthe embodiment 3 of the invention. In addition to the components of theembodiment 1, a torque phase lead reducer 16 is disposed on a stagesucceeding the torque HPF 8 in the second rotating speed estimator 19,and a current phase lead reducer 17 is disposed on a stage succeedingthe current HPF 9 therein.

The characteristic F of the torque HPF 8 demonstrates, as shown in FIG.6, that a low-frequency component of a steering torque signal is cut atan oscillation occurrence frequency but the phase of the steering torquesignal is caused to lead at a steering wheel oscillation frequency.Consequently, the torque phase lead reducer 16 having, as shown as acharacteristic G in FIG. 7, a low-pass filter characteristic that causesthe ratio between the steering wheel oscillation frequency f_(vib) andthe cutoff frequency f_(T HPF) of the torque HPF 8 and the ratio betweenthe cutoff frequency f_(T) _(—) _(comp) of the torque phase lead reducer16 and the steering wheel oscillation frequency f_(vib) to be identicalto each other is disposed on the stage succeeding the torque HPF 8.Moreover, the current phase lead reducer 17 having the same frequencycharacteristic as the torque phase lead reducer 16 does is disposed onthe stage succeeding the current HPF 9. In the invention, the rotatingspeed estimation means encompasses as an example thereof the secondrotating speed estimator.

Next, a processing algorithm in the microcomputer of the embodiment 3will be described below in conjunction with the flowchart of FIG. 8.

First, at step S301, a torque sensor output that is A/D-converted by thetorque A/D converter 3 and is analog-phase-compensated is read andstored in the memory. At step S302, a current detection value that isA/D-converted by the current A/D converter 5 is read and stored in thememory. At step S303, the torque controller 6 reads the torque sensoroutput that is stored in the memory and is analog-phase-compensated,performs mapping computation on an auxiliary torque current, and storesthe resultant current in the memory. At step S304, the torque oppositephase compensator 7 reads the torque sensor output that is stored in thememory and is analog-phase-compensated, performs opposite phasecompensation computation, and stores the torque sensor output, which hasundergone opposite phase compensation computation, in the memory. Atstep S305, the torque HPF 8 performs high-pass filter computation on thetorque sensor output that is stored in the memory and has undergoneopposite phase compensation computation, and stores the torque sensoroutput, which is high-pass-filtered, in the memory. At step S306, thecurrent HPF 9 performs high-pass filter computation on the currentdetection value that is stored in the memory, and stores the currentdetection value, which is high-pass-filtered, in the memory. At stepS307, the torque phase lead reducer 16 performs low-pass filtercomputation on the torque sensor output that is stored in the memory andis high-pass-filtered, and stores the torque sensor output, which islow-pass-filtered, in the memory. At step S308, the current phase leadreducer 17 performs low-pass filter computation on the current detectionvalue that is stored in the memory and is high-pass-filtered, and storesthe current detection value, which is low-pass-filtered, in the memory.At step S309, the motor rotation observer 10 performs motor rotatingspeed computation using the torque sensor output, which is stored in thememory and is low-pass-filtered, and the current detection value that islow-pass-filtered, and stores the result in the memory. At step S310,the damping controller 11 performs damping current computation bymultiplying the motor rotating speed, which is stored in the memory, bya gain, and stores the result in the memory. At step S311, the adder 12adds up an auxiliary torque current and a damping current so as toobtain a target current. At step 312, the current controller 13 performscurrent control computation using the target current and currentdetection value, and outputs the result as a voltage command signal suchas a PWM signal to the drive circuit 14. The processing from step S301to step S312 is executed for every control sampling.

According to the foregoing constitution, since the torque phase leadreducer 16 is disposed on the stage succeeding the torque HPF 8, acharacteristic (total characteristic H in FIG. 9) that is a combinationof the characteristic F of the torque HPF 8 and the characteristic G ofthe torque phase lead reducer 16 can be obtained. As seen from the totalcharacteristic H in FIG. 9, both a gain and a phase are 0s at thesteering wheel oscillation frequency f_(vib). Consequently, a shift canbe eliminated, and the precision in a motor rotating speed signalimproves. As a result, an excellent steering wheel oscillation reductioneffect can be realized.

1-4. (canceled)
 5. An electric power steering control system,comprising: a torque detection means that detects a steering torquecaused by a driver; a torque controller that computes an auxiliarytorque current, which assists the steering torque, using the output ofthe torque detection means; a motor that generates a torque whichassists the steering torque; a current detection means that detects acurrent value to be conducted to the motor; a rotating speed estimationmeans that estimates the rotating speed of the motor; and a dampingcontroller that computes a damping current, which is added to theauxiliary torque current, using the estimate value of the rotating speedof the motor estimated by the rotating speed estimation means, wherein:the rotating speed estimation means includes: a motor current steeringcomponent removal means that removes a component caused by steering fromthe output of the current detection means; a torque steering componentremoval means that removes a component caused by steering from theoutput of the torque detection means; and a rotating speed observer thatcomputes the estimate value of the rotating speed of the motor using thecurrent which has the steering component thereof removed by the motorcurrent steering component removal means, and the steering torqueoutputted from the torque steering component removal means; and at leastone of a motor current phase lead reducer that diminishes a phase leadcaused by the motor current steering component removal means, and atorque phase lead reducer that diminishes a phase lead caused by thetorque steering component removal means is included.
 6. The electricpower steering control system according to claim 5, comprising an analogphase compensator, a torque signal converter, and a torque oppositephase compensator.
 7. The electric power steering control systemaccording to claim 5, wherein the current phase lead reducer and torquephase lead reducer are formed with low-pass filters.
 8. The electricpower steering control system according to claim 7, wherein the currentphase lead reducer has a low-pass filter characteristic causing theratio between a steering wheel oscillation frequency and the cutofffrequency of the current steering component removal means, and the ratiobetween the cutoff frequency of the current phase lead reducer and thesteering wheel oscillation frequency, to be identical to each other; andthe torque phase lead reducer has a low-pass filter characteristiccausing the ratio between the steering wheel oscillation frequency andthe cutoff frequency of the torque steering component removal means, andthe ratio between the cutoff frequency of the torque phase lead reducerand the steering wheel oscillation frequency, to be identical to eachother.